The name of the barge is Resolution and according to the link below it arrived last night and will be here until Friday. It's just unfurled huge crane from it's decks, but I can't see from where I'm sat exactly what its doing!
A quick scoot around the internet suggests that it's a very large "Jack up barge" that is used for onshore and offshore construction. Essentially, they work by moving the large piles (the six pillars) up and down until they are secure on the sea/river bed to provide a stable platform to work from.
There are a couple of pictures and some background information here
It looks as though it was built to install wind turbines in the Thames estuary, and only recently came into service; it's probably going to be a regular feature up and down the Thames for a little while.
Edited 1 times. Last edit at 7 February 2005 5.23pm by Craig.